Basin and Range

In the Tertiary era, the new formations west of the old Colorado Plateau were shot through with volcanoes. Then, as the North American continental plate pivoted to the southwest, the earth’s crust under this region—which would become the Basin and Range Province—stretched thinner and thinner. In fact, the Great Basin of Utah and Nevada is about twice as wide as it was about 18 million years ago. This stretching has resulted in a much thinner layer of underlying basement rock here than in other parts of the continent, and the entire area is riven by faults where parts of the crust have pulled apart. Given the differential forces at work in the earth’s mantle, sometimes half of a fault would be pushed up to mountain heights while the other half would sink, causing a basin. The spectacular fault-block mountains of the Great Basin result from such parallel rising and falling along fault lines. The most famous instance of this type of formation is the rugged Wasatch Mountains, which rise directly above the basin of the Great Salt Lake.

To the east of this momentous fault-block mountain building, the old Colorado Plateau remained relatively undisturbed. However, within the last 10 million years the entire intermountain region bowed up in a broad arch, elevating the old sandstones of the Colorado Plateau; this corner of Utah has risen 5,000 feet during this time. The rivers that once wound across the surface of eastern Utah were forced to cut ever deeper canyons as the formations rose. The erosive power of the Green, Colorado, San Juan, and other rivers and streams have cut down through hundreds of millions of years of rock.

Ice Age Utah

During the geologically recent Pleistocene era, ice-age mountain glaciers and climatic changes brought an abundance of moisture to Utah. The runoff and meltwater flooded the basins of fault-block mountain ranges, forming enormous lakes. The largest of these was Lake Bonneville, the name given to the ice-age predecessor of the Great Salt Lake. At its greatest extent, Lake Bonneville covered nearly all of northern and west-central Utah and was nearly 900 feet deeper than the current Great Salt Lake. Even at that depth, finding an outlet to the sea was not simple. It was only after the lake waters breached Red Rock Pass in Idaho that the lake found an outlet into the Snake and Columbia River systems, about 16,000 years ago.

After the ice ages ended, about 10,000 years ago, Lake Bonneville diminished in size and dropped below the level necessary to cut through Red Rock Pass, resulting in the saline Great Salt Lake. You can easily see the old lake shorelines along the Wasatch Front, and cities like Logan, Provo, and Salt Lake City sprawl along these stairstep-like ledges. Much of the old lake bottom west of Salt Lake City is salt desert and extremely flat. In the Bonneville Salt Flats, the valley is so flat and unbroken that the curvature of the earth can be seen.

© W.C. McRae and Judy Jewell from Moon Utah, 8th Edition